Laser amplifier, an optical system comprising such a laser amplifier and a method of forming such a laser amplifier

ABSTRACT

A laser amplifier, an optical system comprising such a laser amplifier and a method of forming such a laser amplifier for obtaining polarization independent amplification over a large wavelength region. The optical amplifier comprises an active region that is formed on a semiconductor substrate (6). The active layer has been formed through growing of quantum well layers (13, 14, 15, 16) alternating with barrier layers (12). The well layers comprise well layers of a first type (14, 15, 16) having tensile strain together with or without well layers of a second type having compressive strain (13). At least one of the well layers of one type (16) has been grown to a different width and/or with a different material composition than the other well layers of the same type (14, 15).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of optical systems includingoptical fibers and more particularly to the field of laser amplifiers insuch optical systems as well as to methods of forming such laseramplifiers.

DESCRIPTION OF RELATED ART

Semi-Conductor Laser Amplifiers (SCLA) are expected to be importantcomponents in future optical systems. One important reason for this istheir ability to amplify signals in the optical domain withoutconverting them to the electrical domain. This gives flexibilityregarding bit rates and code formats. Another reason is their smallphysical dimensions and suitability for integration (can for example beused as gating switch elements in optical switch matrices). A simpleSCLA can consist of an anti reflection coated semiconductor laser.

However there exists a problem in these devices, which has to do withthe polarization sensitivity of SCLAs. At the output of an ordinarysingle mode fiber the state of polarization (SOP) is fluctuatingrandomly due to temperature variations and mechanical disturbancesdespite the fact that the laser source has a well defined SOP. Thesimplest form of the SCLA does not have a polarization independentamplification. This means that it is incompatible with ordinary fibersif constant signal levels are required. This is a major drawback.

However there exist some designs for polarization independent laseramplifiers. One simply consists of making the waveguide of the laseramplifier more square. This makes the TE- and TM-modes of the light moreequal. One problem with this approach is however that a smaller linewidth and thicker layer thickness than in conventional laser fabricationhas to be used, which will lower the yield drastically in for example alaser amplifier gate switch matrix. Another problem with the laseramplifiers of this type is that they may saturate when amplifying stronginput signals and thus do not work linearly under these circumstances.

Another approach for polarization independent laser amplifiers, which ismore compatible with standard laser fabrication, make use of structureswith two strained quantum well types, one with compressive strain andone with tensile strain. The strain results when the well layers havecompositions that by themselves do not give lattice constants that arematched to the substrate of the amplifier. The compressive wellscontribute to the TE-gain and the tensile wells contribute mostly to theTM-gain (they however contribute slightly to the TE-gain as well).Another advantage of this approach compared with the previous one isthat the polarization dependency in the solitary SCLA can be tailored tocompensate the polarization dependent losses in the rest of the chip (e.g. in the passive interconnecting waveguides or in waveguide crossoversand y-junctions).

This approach does however have one problem. This is the smallwavelength operation region that is obtained. This is due to the factthat the different kinds of strained quantum wells have differentwavelength dependencies, which limits the effectiveness of the amplifierto a small wavelength region and a system with such amplifiers willtherefore be limited to a small variety of laser sources.

Another problem with these limitations of the amplifier with layershaving strain is that it is difficult to amplify signals with the samegain if they have different wavelengths.

It therefore exists a need within the field of a laser amplifier, whichhas a polarization independent behaviour over a large wavelength regionand which at the same time does not saturate when strong signals areused.

In the article "Effects of nonuniform well width on compressivelystrained multiple quantum well lasers", D Teng et al, Appl. Phys. Lett.,Vol 60 (1992), p 2729-2731 a quantum well laser is described, which hascompressive wells, the width of which have been varied. In the article,which is directed to a laser source and not a laser amplifier, theauthors note that the varied widths in compressively strained wells giverise to a wider wavelength region.

In U.S. Pat. No. 5,363,392 a semiconductor laser device is described,which has quantum wells with tensile strain separated by barrier layerswith compressive strain. The widths or the material compositions of thewells as well as the barriers can be varied. This document is directedtowards problems encountered in laser sources and the aim is to obtain adevice which works well with low threshold currents at hightemperatures. This document does not describe problems concerningamplification of optical signals or the way such problems can be solved.

None of the above mentioned documents concern laser amplifiers which arepolarization independent over a large wavelength region.

SUMMARY OF THE INVENTION

One object of the present invention is to obtain a method of forming alaser amplifier of the quantum well type that can achieve polarizationindependent amplification of optical signals over a large wavelengthregion.

This object is achieved through the use of a method for forming anactive region on a semiconductor substrate in a laser amplifiercomprising growing of well layers alternating with barrier layers. Thewell layers comprise a first type that has tensile strain together withor without a second type that has compressive strain. Of these welllayers at least one of one type is grown to a different width and/orwith a different material composition than the other well layers of thesame type.

Another object of the present invention is to obtain a laser amplifierof the quantum well type that has polarization independent amplificationof optical signals over a large wavelength region.

This object is achieved through a laser amplifier that has an activeregion that comprises quantum wells separated by barriers, which wellscomprise wells of a first type having tensile strain together with orwithout wells of a second type having compressive strain. Of these wellsat least one of one type has a different width and/or different materialcomposition than the rest of the wells of the same type.

Another object is to obtain an optical system that comprises at leastone laser amplifier of the quantum well type that has polarizationindependent amplification of optical signals over a large wavelengthregion.

This is achieved through an optical system that comprises a laseramplifier comprising an active region comprising quantum wells that areseparated by barriers, which wells comprise wells of a first type havingtensile strain together with or without wells of a second type havingcompressive strain. Of these wells in the laser amplifier at least oneof one type has a different width and/or different material compositionthan the rest of the wells of the same type.

With the present invention laser amplifiers are obtained that arecompatible with ordinary fibers when constant signal levels arerequired.

With the present invention a laser amplifier and an optical system arealso obtained where the TM- and TE-modes of any signal that lies withina desired wavelength region are amplified substantially with equal gain.

With the present invention a laser amplifier is obtained that has apolarisation independent amplification over a large wavelength regionthat is substantially better than in quantum well laser amplifiers ofthe prior art.

In the specification the term layer product is used. It is here definedas the width of a layer in the active region of a laser amplifiermultiplied with the strain of said layer, where the strain is expressedin percent. A tensile strain is here defined as having a positive signand a compressive strain is defined as having a negative sign.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing some of the parts of an opticalsystem according to the present invention,

FIG. 2a is a schematic perspective view of a laser amplifier accordingto the prior art,

FIG. 2b is an end view of an encircled part of the laser amplifier inFIG. 2a showing the active region and the layers enclosing it,

FIG. 2c is an enlarged view schematically showing the structure of theactive region in an encircled part of FIG. 2b,

FIG. 2d shows part of the energy diagram for the layers shown in FIG.2c,

FIG. 3 shows the energy diagram for a laser amplifier according to apreferred embodiment of the present invention,

FIG. 4 shows a diagram of the gain in dependence of the photon energy inthe laser amplifier according to the preferred embodiment of the presentinvention,

FIG. 5 shows a diagram of the gain in dependence of the photon energy ina laser amplifier according to the prior art,

FIG. 6 shows curves of the gain in dependence of the photon energy forquantum wells with tensile strain having different widths and,

FIG. 7 shows curves of the gain in dependence of the photon energy forquantum wells with compressive strain having different widths andmaterial compositions.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described with reference tothe attached drawings in the following section.

In FIG. 1 a schematic view of some of the parts of an optical systemaccording to the invention is shown. The optical system comprises alaser source 1, a long optical fiber 2, a semiconductor laser amplifier(SCLA) 3 and yet another fiber 4, which could be connected to yetanother SCLA, an amplifier of another type, a repeater, a receiver etc.(not shown). It should be understood that the different parts of thesystem, as opposed to in the figure, are closely connected to oneanother in order for as much light as possible to travel within thefibers and the amplifier. The laser source 1 emit signals that have acertain wavelength and a well defined state of polarization (SOP). Asthe signals travel through the long fiber 2 the polarization is affectedby temperature variations and mechanical disturbances so that the stateof polarization is fluctuating randomly at the end of the fiber 2 facingthe SCLA 3. The amplifier 3 according to the invention then amplifiesthe input signals independently of the polarization when these signalshave a wavelength within a wavelength region that is quite large andoutputs the amplified signals to the fiber 4.

FIGS. 2a-2d serve the purpose of showing the quantum well structure of aSCLA. In FIG. 2a a known SCLA 3 is shown. In FIG. 2b an end view of apart of the SCLA, which is encircled in FIG. 2a, is shown. The viewshows the active region 5, which is grown on a substrate 6 andsurrounded by blocking layers 8 and 9. On top of the active region 5 acontact layer 11 is disposed. The active region 5 could also have hadcladdings or other types of confinement layers included between thequantum well region and the substrate and the quantum well region andthe contact layer 11. In this figure the growth direction is designatedz. An injection current is also shown supplied to the contact layer foroperating the amplifier.

In FIG. 2c an enlarged view of the active region 5 that is encircled inFIG. 2b is shown. The layers in the active region are stacked along thegrowth direction z with alternating well layers 30 and barrier layers32. The well layers 30 in this known active region 5 can be of a firsttype having tensile strain and/or of a second type having compressivestrain, and all the well layers of the first type have the same width,i.e. 10 nm, and all the layers of the second type have the same width,that can be different from the width of the well layers of the firsttype. All the barrier layers 32, perhaps with the exception for theinnermost layer facing the substrate and the outermost layer facing thecontact layer 11, have the same width, i.e. 10 nm.

FIG. 2d shows the energy diagram for the band edges of the conductionband corresponding to layers in FIG. 2c, with the quantum wells andbarriers stacked along the growth direction z. The valence band isomitted in this figure. Amplification takes place in a wavelength regionthat mainly is decided by the bandgap between the conduction band edgesand the valence band edges of the quantum wells 30, but this region isalso influenced by the width and the material composition of the quantumwells 30, as will be described later.

In FIG. 3 the energy diagram for a preferred embodiment of the presentinvention is shown. The structure according to FIGS. 2a and 2b is alsoapplicable for this structure. The laser amplifier according to theinvention is made in the InGaAsP material structure. However othermaterial structures are conceivable.

The structure according to the preferred embodiment of the inventionwill be explained with reference to the upper part of FIG. 3, whichshows the conduction band. The structure comprises an active regionbetween a substrate 6 and a contact layer 11 both of InP. The activeregion, which is made of the In_(1-x) Ga_(x) As_(Y) P_(1-y) materialsystem comprises quantum wells 13 of the second type having compressivestrain and wells 14, 15 and 16 of the first type having tensile strain.The wells 13 of the second type are three and all have a width of 7 nmand material composition parameters x=0.13 and y=0.72. Two wells 14 ofthe first type have a width of 20 nm and material composition parametersx=0.55 and y=1, two wells 15 of the first type have a width of 15 nm andmaterial composition parameters x=0.55 and y=1 and two wells 16 of thefirst type have a width of 10 nm and material composition parametersx=0.55 and y=1. The wells are separated by barriers 12 having a width of15 nm and material composition parameters x=0.12 and y=0.25. Twoadditional wider barrier layers 10 with the same material compositionparameters are provided between the wells and the substrate 6 and thewells and the contact layer 11, respectively, which gives a total widthof the active region of approximately 0.3 μm.

The active region was formed in the following way according to a methodaccording to the invention. First the wide barrier 10 was grown on thesubstrate 6. On top of this wide barrier 10 quantum wells 13, 14, 15, 16were grown alternating with barriers 12. The barriers 12 were all grownto a width of 15 nm. The well layers were grown in the following way andorder in the growth direction z: a well of the second type 13 to a widthof 7 nm, a well of the first type 16 to a width of 10 nm, a well of thefirst type 14 to a width of 20 nm, a well of the second type 13 to awidth of 7 nm, a well of the first type 15 to a width of 15 nm, a wellof the first type 15 to a width of 15 nm, a well of the second type 13to a width of 7 nm, a well of the first type 14 to a width of 20 nm anda well of the first type 16 to a width of 10 nm. On top of this lastwell 16 of the first type a second wider barrier 10 was grown andfinally a contact layer was formed on the second wide barrier 10.

The lower part of the diagram in FIG. 3 shows the energy levels of thestructure for the valence band of the active layer. For each well thereexist two different energy levels, one for light holes, shown withdashed lines, and one for heavy holes, shown with solid lines. Thesedifferent levels occur because of the strain, which is well known forthe person skilled in the art.

FIG. 4 shows the gain of the TE- and TM-modes in dependence of thephoton energy for the laser amplifier with the structure according toFIG. 3. The TM-mode is shown with a dashed line and the TE-mode is shownwith a solid line. The gain is expressed in cm⁻¹ and the photon energy,which is inversely proportional to the wavelength, in eV. As acomparison FIG. 5 shows the gain in the same photon energy region(corresponding to the desired wavelength region) in a laser amplifieraccording to the prior art having a structure of four wells of thesecond type, each having a width of 7 nm and material compositionparameters x=0.13, y=0.72, and five wells of the first type, each havinga width of 20 nm and material composition parameters x=0.55 and y=1. Inthe figure the TM-mode gain is also shown with a dashed line and theTE-mode gain in a solid line.

As can be seen from FIGS. 4 and 5 the amplification in the laseramplifier according to the invention has a more even amplification overthe desired wavelength region than the laser amplifier according to theprior art. The TE- and TM-modes are also amplified with substantiallyequal gain in the wavelength region by the amplifier according to theinvention.

This can also be expressed through the equation below:

    max[(g.sub.TE -g.sub.TM)/(g.sub.TE +g.sub.TM)]             (1)

The maximum value according to above equation is calculated as 0.11 forthe laser amplifier of the prior art and as 0.044 for the laseramplifier according to the invention, which is a considerableimprovement of more than 100%.

In the embodiment of the invention shown above the active regionincluded wells of both the first and the second type separated bybarrier layers. In another embodiment of the laser amplifier the activeregion only includes wells of the first type with just a slight strain(a few tenths of a percent) that are separated by barrier layers.Moreover, in the described preferred embodiment of the invention onlythe widths of the wells of the first type have been varied. The materialcompositions of said wells could just as well have had been varied aswell as a combination of width variation and material compositionvariation. The compressive wells could also have had varied widthsand/or material compositions. Finally the active region could have hadmore or fewer wells in the structure, both in number of wells of thefirst type as well as wells of the second type.

In order to further clarify how variations of width and materialcomposition can be made in laser amplifiers according to the invention,reference is being made to FIGS. 6 and 7. FIGS. 6 and 7 are curves thatshow the gain contributions obtained through choice of materialcomposition and width of the wells.

FIG. 6 shows the contributions 17, 18 and 19 to the amplification of theTE- and TM-modes from quantum wells of the first type having widths of20 nm, 15 nm and 10 nm, respectively and material parameters x=0.55 andy=1. The contributions to the TE-gain is shown with dashed lines and thecontributions to the TM-gain with solid lines. As can be seen the wellsof the first type mainly contribute to the TM-gain, but a certaincontribution to the TE-gain is also achieved. As is also apparent fromthe curves, the different widths result in gain peaks for differentwavelengths and the larger widths contribute mostly to the lower photonenergy levels and the smaller widths to the higher photon energy levels.A change of material composition (not shown) also changes the gainpeaks. A slight increase of the x-parameter (say from 0.55 to 0.56) anda slight decrease of the y-parameter (say from 1 to 0.98) would give again contribution at a higher photon energy level, and a slight decreaseof the x-parameter (say from 0.55 to 0.54), with the y-parameter kept aty=1 (can not be more than 1) would give a contribution to a lower photonenergy level.

FIG. 7 shows the gain contributions from the wells of the second type.Here the contributions from a well 20 having a width of 7 nm andmaterial parameters x=0.13 and y=0.72, a well 21 having a width of 7 nmand material parameters x=0.15, y=0.70 and a well 22 having a width of 6nm and material parameters x=0.15, y=0.70 are shown. As can be seen fromthe figure the contributions to the TM-mode for these wells is almostnegligible. The different widths also give gain peaks at differentwavelengths, where the larger widths give peaks at lower photon energylevels than at smaller widths. A higher x-parameter and a lowery-parameter also shifts the peaks to a higher photon energy level.

As can thus be seen from FIGS. 6 and 7, a gain peak is shifted towardslower photon energy levels by increasing the width of a well and viceversa. The material composition can be varied in the same way.

In order to achieve polarization independent amplification in a desiredwavelength region one picks a known laser amplifier structure thatamplifies well in say the middle of the desired region and then widthsand/or material compositions of the quantum wells are varied in abovementioned manner to obtain the desired wavelength region.

However there exists certain limits for this active region. The numberof wells that can be included in an active region according to theinvention is restricted in the following way.

The absolute value of a layer product, which is defined as the width ofa layer multiplied with the strain of said layer, is less than 20 nmpercent, when the width is expressed in nm and the strain in percent. Inaddition to this the following requirement must be fulfilled. Theabsolute value of any sum of layer products for consecutive layers isless than 20 nm percent. The tensile strain is here defined as having apositive sign and the compressive strain as having a negative sign,although the opposite signs might just as well have been chosen. Thismeans that no absolute value of any sum of layer products t₁ s₁, t₁ s₁+t₂ s₂, . . . ,t₁ s₁ +t₂ s₂ + . . . +t_(n) s_(n) for n consecutivelayers can be more than 20 nm percent. In above expressions t_(n)indicates the width of a layer and s_(n) the strain of said layer.

For the structure according to the preferred embodiment of the presentinvention the layer products are as follows.

The wells 13 of the second type have a strain of about -1.59 percent,the wells 14, 15, 16 of the first type have a strain of about 0.45% andthe barrier layers 10, 12 lack strain.

The layer product for each well 13 of the second type is then7*(-1.59)=-11.13 nm percent and the layer products for the wells of thefirst type 14, 15, 16 are then 20*0.45=9 nm percent, 15*0.45=6.75 nmpercent and 10*0.45=4.5 nm percent, respectively. The layer products ofthe barriers are all zero since they lack strain. As can be seen all thelayer products meet above mentioned requirement.

As can thus be seen through summing the layer products of anycombination of consecutive layers in the active region, the absolutevalue of any such sum is always less than 20 nm percent.

I claim:
 1. A method for forming a laser amplifier comprising:forming ofactive region on a semiconductor substrate, the forming of the activeregion comprising growing of well layers alternating with barrierlayers, the well layers comprising well layers of a first type havingtensile strain together with or without well layers of a second typehaving compressive strain, wherein at least one of the well layers ofone type is grown to a different width and/or with a different materialcomposition than the other well layers of the same type.
 2. The methodof claim 1, wherein the well layer of one type that differs from theother well layers of the same type is grown in such a way that a gainenhancement is achieved in at least one wavelength region that at leastpartly differs from the wavelength region where the gain peak for theother well layers of the active region is located.
 3. The method ofclaim 1, wherein at least one first well layer of one type is grown to awidth that is about half the width of the other well layers of the sametype.
 4. The method of claim 3, wherein at least one second well layerof the same type as the first well layer, is grown to a width betweenthe width of the first well layer and the width of the other well layersof the same type.
 5. The method of claim 1, wherein each layer in theactive region is grown without strain or with such a strain and to sucha width that the absolute value of a layer product for said layer isless than 20 nm percent, the layer product being the width of a layermultiplied with the strain of said layer and the width being expressedin nm and the strain in percent.
 6. The method of claim 5, wherein thelayers in the active region are grown without strain or with such astrain and to such a width that the absolute value of any sum of layerproducts for consecutive layers is less than 20 nm percent.
 7. A laseramplifier with an active region comprising:quantum well layers separatedby barrier layers, the well layers comprising well layers of a firsttype having tensile strain together with or without well layers of asecond type having compressive strain, wherein at least one of the welllayers of one type has a different width and/or a different materialcomposition than the other well layers of the same type.
 8. The laseramplifier of claim 7, wherein the well layer of one type that differsfrom the other well layers of the same type is chosen in such a way thata gain enhancement is achieved in at least one wavelength region that atleast partly differs from the wavelength region where the gain peak forthe other well layers of the active region is located.
 9. The laseramplifier of claim 7, further comprising:at least one first well layerof one type and with differing width, the width being about half thewidth of the other well layers of the same type.
 10. The laser amplifierof claim 9, further comprising:at least one second well layer withdiffering width and of the same type as the first well layer, the widthof the second well layer being between the width of the first well layerand the width of the other well layers of the same type.
 11. The laseramplifier of claim 7, wherein the absolute value of a layer product fora layer in the active region, comprising the width of the layermultiplied with the strain of said layer, is less than 20 nm percent,where the width is expressed in nm and the strain in percent.
 12. Thelaser amplifier of claim 11, wherein the absolute value of any sum oflayer products for consecutive layers is less than 20 nm percent.
 13. Anoptical system comprising:a laser source; at least one optical fiber;and at least one laser amplifier, the laser amplifier comprising anactive region comprising quantum well layers separated by barrierlayers, the well layers comprising well layers of a first type havingtensile strain together with or without well layers of a second typehaving compressive strain, wherein at least one of the well layers ofone type in the laser amplifier has a different width and/or a differentmaterial composition than the other well layers of the same type in thelaser amplifier.
 14. The optical system of claim 13, wherein the welllayer of one type that differs from the other well layers of the sametype in the laser amplifier is chosen in such a way that a gainenhancement is achieved in at least one wavelength region that at leastpartly differs from the wavelength region where the gain peak for theother well layers in the laser amplifier is located.
 15. The opticalsystem of claim 13, further comprising:at least one first well layer ofone type and with differing width in the laser amplifier, the widthbeing about half the width of the other well layers of the same type inthe laser amplifier.
 16. The optical system of claim 15, furthercomprising:at least one second well layer with differing width in thelaser amplifier and of the same type as the first well layer, the widthof the second well layer being between the width of the first well layerand the width of the other well layers of the same type in the laseramplifier.
 17. The optical system of claim 13, wherein the absolutevalue of a layer product for a layer in the active region of the laseramplifier, comprising the width of a layer multiplied with the strain ofsaid layer, is less than 20 nm percent, the width being expressed in nmand the strain in percent.
 18. The optical system of claim 17, whereinthe absolute value of any sum of layer products for a laser amplifier isless than 20 nm percent for any consecutive layers.
 19. A method forforming a laser amplifier comprising:forming an active region on asemiconductor substrate, the forming of the active region comprisinggrowing of well layers alternating with barrier layers, the well layerscomprising well layers of a first type having tensile strain togetherwith or without well layers of a second type having compressive strain;wherein at least one of the well layers of one type is grown to adifferent width or with a different material composition than the otherwell layers of the same type.
 20. A method for forming a laser amplifiercomprising:forming an active region on a semiconductor substrate, theforming of the active region comprising growing of well layersalternating with barrier layers, the well layers comprising well layersof a first type having tensile strain together with or without welllayers of a second type having compressive strain; wherein at least oneof the well layers of one type is grown to a different width and with adifferent material composition than the other well layers of the sametype.
 21. A laser amplifier with an active region comprising:quantumwell layers separated by barrier layers, the well layers comprising welllayers of a first type having tensile strain together with or withoutwell layers of a second type having compressive strain; wherein at leastone of the well layers of one type has a different width or a differentmaterial composition than the other well layers of the same type.
 22. Alaser amplifier with an active region comprising quantum well layersseparated by barrier layers, the well layers comprising well layers of afirst type having tensile strain together with or without well layers ofa second type having compressive strain;wherein at least one of the walllayers of one type has a different width and a different materialcomposition than the other well layers of the same type.
 23. An opticalsystem comprising:a laser source; at least one optical fiber; and atleast one laser amplifier, the laser amplifier comprising an activeregion comprising quantum well layers separated by barrier layers, thewell layers comprising well layers of a first type having tensile straintogether with or without well layers of a second type having compressivestrain; wherein at least one of the well layers of one type in the laseramplifier has a different width or a different material composition thanthe other well layers of the same type in the laser amplifier.
 24. Anoptical system comprising:a laser source; least one optical fiber; andat least one laser amplifier, the laser amplifier comprising an activeregion comprising quantum well layers separated by barrier layers, thewell layers comprising well layers of a first type having tensile straintogether with or without well layers of a second type having compressivestrain; wherein at least one of the well layers of one type in the laseramplifier has a different width and a different material compositionthan the other well layers of the same type in the laser amplifier. 25.A method for forming a laser amplifier comprising:forming an activeregion on a semiconductor substrate, the forming of the active regioncomprising growing of well layers alternating with barrier layers, thewell layers comprising well layers of a first type having tensile strainand well layers of a second type having compressive strain; wherein atleast one of the well layers of said first type or said second type isgrown to a different width or with a different material composition thanother well layers of the same type.
 26. A laser amplifier with an activeregion comprising:quantum well layers separated by barrier layers, thewell layers comprising well layers of a first type having tensile strainand well layers of a second type having compressive strain; wherein atleast one of the well layers of said first type or said second type hasa different width or a different material composition than other welllayers of the same type.